In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, for future generations of mobile communications systems frequency bands at many different carrier frequencies could be needed. For example, low such frequency bands could be needed to achieve sufficient network coverage for users (e.g. wireless devices) and higher frequency bands (e.g. at millimeter wavelengths (mmW), i.e. near and above 30 GHz) could be needed to reach required network capacity. In general terms, at high frequencies the propagation properties of the radio channel are more challenging and beamforming both at the network-side (e.g. at transmission points or access nodes) and at the user-side might be required to reach a sufficient link budget.
At the transmission points hundreds of antennas elements are expected to be used for beamforming in order to counteract poor radio channel propagation properties.
In the document “Echo-MIMO: A Two-Way Channel Training Method for Matched Cooperative Beamforming”, by Lang P. Withers, Jr., in IEEE Transactions on Signal Processing, Vol. 56, No. 9, September 2008, there is a description of full loop precoding for a wireless device. The purpose of the precoding scheme is to attain channel state information (CSI) in the uplink from the wireless device when the wireless device is not calibrated between transmission and reception. The CSI can then be used at the wireless device for uplink precoding. A simplified signaling diagram of the method described in this document is given in FIG. 1.
Step S1: The wireless device 300 transmits sounding reference signals (SRSs) and the network node 200 receives the transmitted SRSs.
Step S2: The network node 200 amplifies and forwards the received SRSs to the wireless device 300 and the wireless device 300 receives the forwarded SRSs.
Step S3: The wireless device 300 performs an estimation of the round-trip channel HRound using the received SRSs.
Step S4: The network node 200 transmits downlink reference signals (DL-RSs) and the network node 200 receives the transmitted DL-RSs.
Step S5: The wireless device 300 performs an estimation of the downlink channel HDL using the received DL-RSs.
Step S6: The wireless device 300 performs an estimation of the uplink channel HUL by performing element-wise matrix division between HRound and HDL.
One issue with the full loop uplink precoding procedure in FIG. 1 is that if the network node 200 has many antennas it generates large overhead, especially due to the forwarded uplink SRSs. The number of forwarded SRSs is equal to M times N, where M is the number of SRSs transmitted from the wireless device (typically given by the number of antennas at the wireless device) and N is the number of receive antennas at the transmission point of the network node. There is also extra overhead due to the required N extra downlink reference signals.
Hence, there is still a need for improved channel estimation procedures.